Acid-gold electroplating bath



United States Patent Office 3,367,853 Patented Feb. 6, 1968 3,367,853 ACID-GOLD ELECTROPLATING BATH Karl Schumpelt, Union, N.J., assignor to Sel-Rex Corporation, Nutley, N.J., a corporation of New Jersey No Drawing. Continuation-impart of application Ser. No. 424,496, Dec. 31, 1964. This application Aug. 26, 1965, Ser. No. 482,903

7 Claims. (Cl. 20446) This application is a continuation-in-part of US. application Ser. No. 424,496, filed Dec. 31, 1964, now abandoned.

This invention relates to the art of metal coating With particular reference to a process and method for electrodepositing gold.

In years past it was customary to electrodeposit gold from an aqueous solution containing an alkali gold cyanide and some excess alkali cyanide with or without additional conducting salts. Such solutions Were well on the alkaline side. About the middle of the twentieth century several gold plating baths, sans free cyanide, were invented, and these were operated at what was essentially a neutral pH. About ten years later, Rinker and Duva discovered that potassium gold cyanide was stable at much lower pH values than theretofore known. These baths, as disclosed in U.S. Patents Nos. 2,905,601 and 3,104,212, contain 1- 30 g./l. of gold added as alkali metal gold cyanide and -150 g./l. of an organic acid partially neutralized to provide a pH of 3-6.

Among the objects of this invention is to provide a bath which makes it possible to plate from the aforementioned acid gold solution, a gold deposit characterized by being harder and more abrasion resistant.

Another object is to provide a method for obtaining a deposit from an acid gold bath which is smoother and somewhat brighter than deposits obtained from the bath described in US. Patent No. 3,104,212.

This invention is based on my discovery that the addition of an alum to a gold bath containing a weak organic acid partially neutralized with sodium, potassium, or ammonium hydroxide, or a combination of these alkalies, to provide a pH of 3.06.0 modifies the structure of the gold electrodeposit obtained therefrom to give a finer grained texture which is also harder. The amount of alum is not very critical, 1 to 100 g./l. having been found useful. It is preferred, however, to add 10 to grams to each liter of plating solution.

The term alum will be understood to define compounds such as are obtained by crystallizing the sulfates of aluminum, chromium and gallium with the sulfates of ammonium, cesium, potassium, rubidium, sodium and thallium and with Water in which the molar ratio,

M22804: M2 (SO4)3 i H2O is 1:1:24, where M is the monovalent metal and M is the trivalent metal.

Surprisingly, I have been unable to detect any chromium, aluminum, or gallium in deposits prepared in such a gold plating solution even though the increased hardness would indicate some codeposition. Apparently, the trivalent metal is complexed in such a manner as not to be plated out with the gold. On the other hand, if trivalent metal sulfate is added to the bath the deposit becomes very bright and hard. Chromium, for example, can easily be identified by X-ray fluorescence.

The unexpected stability of these alums in this acid gold plating system leads to the production of a high-purity gold deposit with improved physical properties with respect to hardness and smoothness.

It will be understood that the buffered acid solutions of the invention can be obtained by mixing free acid and neutralizing alkaline salts or by dissolving acid reacting salts of the weak acids to form the solution or by mixing the free acid and salts of the free acid. The acidic nature of the bath is denoted by its pH of 3 to 6.5.

The following examples will explain the composition of the bath and method of plating.

Example 1 A bath was prepared as follows:

Gold (added as KAu(CN) g./l 8.2 Ammonium citrate ((NHQ C H O g./l 50 Chrome alum (NH C1'(SO .12H O) g./l 20 pH 5.0

A brass panel 3 /2 x 1" was plated at 0.5 amp/dm. at 50 C. to a thickness of about 25 microns. The deposit was smooth and semi-bright. The hardness was found to be kg./mm. (Knoop with 25 g. load).

Example 2 A bath was prepared containing:

Gold (added as KAu(CN) g./l 16.5 Ammonium citrate ((NHg C H O g./l 50 Chrome alum (NH Cr(SO .12H O) g./l 50 pH 5.5

A brass panel 3 /2 x 1 was plated at 1.0 arnp/dm. at 50 C. to a thickness of 20 microns. The hardness was 118 kg./rnm. Knoop at 25 g. load.

In other experiments the ammonium citrate was varied from 10 to grams per liter and various amounts of sodium and potassium citrate were added with no apparent effect on the deposit. The acid or acid anion need not be citric or citrate but may be any stable organic acid with a pK value such that the acid will be an effective buffer in the pH range of about 3 to about 6.5.

KOH to provide a pH 5.0

A brass panel 3 /2" x 1" Was plated at 10 amp/dm. at 50 C. to a thickness of 5 microns. The hardness was 110 kg./mm. and the product was smooth and semi bright.

Panels plated with the aluminum alums and mixtures of the aluminum, chromium and gallium alums show substantially the same brightness and hardness.

The features and principles underlying the invention described above in connection with specific exemplifications will suggest to those skilled in the art many other modifications thereof. It is accordingly desired that the appended claims shall not be limited to any specific feature or details thereof.

I claim:

1. An aqueous electrolyte for electrodepositing smooth, hard, 24-karat gold containing about 130 g./l. of gold added as a soluble alkali metal gold cyanide, about 10 150 g./l. of a stable, organic acid selected from the class having a pK value effectively buffering at a pH of 3 to 6.5, partially neutralized with an alkaline material to yield a salt of said acid selected from the class consisting of sodium, potassium, lithium and ammonium salts and combinations thereof and to provide a pH of 3 to 6.5, and 1 to 100 g./l. of an alum.

2. An electrolyte as claimed in claim 1 in which said organic acid is citric acid.

3. An electrolyte as claimed in claim 1 in which said alum is a compound of the formula Where M is a monovalent cation selected from the group consisting of ammonium, cesium, potassium, rubidium, sodium and thallium and M is a trivalent metal selected from the group consisting of aluminum, chromium and gallium.

4. An electrolyte as claimed in claim 1 in which said alum is alkali metal chromium disulfate decahydrate.

5. An electrolyte as claimed in claim 1 in which said alum is ammonium chromium disulfate decahydrate.

References Cited UNITED STATES PATENTS 8/1880 Lisenmayer 204-43 9/1963 Rinker et al. 20446 HOWARD S. WILLIAMS, Primary Examiner.

G. KAPLAN, Assistant Examiner. 

1. AN AQUEOUS ELECTROLYTE FOR ELECTRODEPOSITING SMOOTH, HARD, 24-KARAT GOLD CONTAINING ABOUT 1-30 G./1. OF GOLD ADDED AS A SOLUBLE ALKALI METAL GOLD CYANIDE, ABOUT 10150 G./1. OF A STABLE ORGANIC ACID SELECTED FROM THE CLASS HAVING A PK VALUE EFFECTIVELY BUFFERING AT A PH OF 3 TO 6.5, PARTIALLY NEUTRALIZED WITH AN ALKALINE MATERIAL TO YIELD A SALT OF SAID ACID SELECTED FROM THE CLASS CONSISTING OF SODIUM, POTASSIUM, LITHIUM AND AMMONIUM SALTS AND COMBINATIONS THEREOF AND TO PROVIDE A PH OF 3 TO 6.5, AND 1 TO 100 G./L. OF AN ALUM. 